The dual problems of environmental air pollution and poor automotive fuel efficiency have forced the U.S. Government to legislate maximum limits on the amounts of CO, HC and NOx a vehicle can emit into the atmosphere, and to set long range average fleet MPG requirements for each auto manufacturer.
The U.S. Government is presently seeking to further restrict tail pipe emissions while further increasing fuel economy. It is also proposing alternative fuels for automobiles, such as alcohol, LPG and gas-alcohol mixtures to reduce emissions and decrease U.S. dependency on foreign oil. State and local governments are passing even stricter emission laws and are even planning to restrict the use of internal combustion engines in certain localities.
Auto manufacturers have attempted to answer government proposed emissions controls requirements in the past by utilizing a three-way catalytic converter in automotive exhaust systems to burn the pollutants emitted by the engine and thereby meet governmental emission standards. Emissions are legally monitored by the EPA, which tests all new cars for emissions and MPG, and also monitors the exhaust emissions of cars on the road.
The catalytic converter burns the pollutants efficiently only when the engine is running at or near an air-fuel ratio of 14.7:1, the stoichiometric or chemically correct ratio. When this ratio of air and fuel is provided in a mixture and the mixture is ignited, all of the carbon and hydrogen completely burns, yielding only carbon dioxide and water in the exhaust (if combustion were perfect).
However, experience has shown that 14.7 to 1 is functionally ideal only in steady-state operation (such as in turnpike cruising) which involves only minute variations in throttle valve angle, manifold pressure and engine speed. A leaner mixture with a ratio of 16.0:1 or greater has been found adequate for part-throttle, light-load operation, but a richer mixture approaching 12:1 is required for full-throttle acceleration. A ratio of 10:1 or 11:1 is preferable for a hot engine at idle, but for cold starts the mixture must be as rich as 3:1 or 4:1, because of the poor atomization of fuel in cold air. Thus, it is generally impossible to run an automobile engine with the same air-to-fuel ratio for all engine speeds and operating parameters. Designers and manufacturers of air and fuel delivery systems (e.g., carburetors and fuel injection systems) must generally make a number of compromises to provide acceptable engine operation over the entire range of engine operating conditions.
The problem of controlling engine air-fuel mixture over a range of operating conditions is exacerbated by the problem of incomplete combustion.
Normal combustion occurs when the mixture in the combustion chamber is ignited by a spark plug firing at a preset point in time near the end of the compression stroke in a four-stroke engine, starting a wave of flame spreading out from the spark plug. This flame front continues to move through the combustion chamber until it reaches the other side. The compressed mixture burns smoothly and evenly. However, flame-front speed varies in speed--from twenty feet per second to over 150 feet per second, depending mainly on air-fuel ratio, compression ratio, turbulence and combustion chamber design. Flame travel is quite slow when the mixture is very rich, and is also slow when the mixture is very lean.
Thus, the ratio by weight of air to fuel in the air-fuel mixture determines the degree to which the mixture can burn and combustion can proceed. A lean mixture contains excess air and, as the mixture becomes leaner, it takes longer to complete the burn of the mixture. On the other hand, a rich mixture does not contain enough air to burn the mixture completely. At very rich conditions, the mixture fails to burn sufficiently to produce adequate power, leaving a large amount of the fuel unburned in the exhaust of the engine.
But even when supplied with a fuel mixture at or near a stoichiometric ratio, engine operating parameters may not allow time for complete combustion. Conventional wisdom in the art is that more or less complete combustion is obtained so long as the mixture for combustion is within a range near stoichiometry (e.g., within the range of about 51/2 or 6 percent by weight of the ideal 14.7:1 stoichiometric ratio for gasoline). However, even though such range of mixtures will burn more or less completely given enough time, mixtures not exactly at the ideal stoichiometric ratio may nevertheless burn too slowly to burn completely under some engine operating conditions. Adjustment of ignition timing with engine rpm and control of other factors can reduce but not entirely eliminate this incomplete burning.
Consequently, even a charge that is theoretically in or near perfect balance will normally leave some unburned fuel in the combustion chamber in the form of raw hydrocarbons and carbon monoxide. These are expelled in the exhaust, and on most post-1975 cars are treated by the catalytic converter that breaks them down chemically into harmless constituents before letting them escape into the atmosphere.
Near the lean limit of engine operation, hydrocarbon and carbon monoxide emissions are at their minimum. Nitrous oxide emissions are highest at the stoichiometric ratio, and fall off toward both the rich and lean mixture extremes. And, of course, fuel economy is increased if the engine is oxide) is perhaps much more harmful to the environment than either HC or CO--and the way of the future may therefore be to run engines with much leaner mixtures so as to reduce NOx emissions and increase fuel economy.
Unfortunately, significant problems arise when one attempts to run a conventional internal combustion engine with a very lean mixture, since the lean mixture generally burns too slowly to fully combust. Although it is becoming increasingly desirable to operate an engine as lean as possible to increase fuel efficiency without exceeding legal emission levels, there is a risk of early "flameout" (i.e., the failure of the mixture in the combustion chamber to burn) with overly-lean mixtures, i.e., air-fuel ratios greater than 16:1. This early "flameout" also increases hydrocarbon and carbon monoxide emissions to unacceptable levels. Leaning out the mixture even more can lead to a situation in which ignition cannot occur at all, causing missed combustion events and stalling.
One approach used in the past to improve the performance of lean-burn automotive engines is to use a so-called "pre-chamber".
An exemplary pre-chamber comprises a smaller secondary combustion chamber, usually housing the spark plug, which communicates with the main combustion chamber. Pre-chambers in internal combustion engines are well known for the purpose of initially spark-igniting a relatively rich air-fuel mixture within the pre-chamber. The ignited mixture then "torches out" to more rapidly ignite a relatively leaner mixture in the main combustion chamber. The flame front emanating from the pre-chamber has a large surface area and therefore very efficiently and rapidly ignites the lean mixture within the main combustion chamber.
Such pre-chamber arrangements have been used in the past in an attempt to increase fuel efficiency. It is know to retro-fit a pre-chamber to an existing engine design by screwing it into the existing spark plug hole of each cylinder, and pre-chambers machined or otherwise provided integral with the engine cylinder head or other components are also known.
As mentioned above, one operational feature associated with pre-chamber operation is that the energy released in the "torch" emanating from the pre-chamber combusts the main chamber air-fuel mixture much faster than the mixture would ignite without the pre-chamber. Conventional ignition systems utilized on most automotive engines apply 70 to 120 millijoules of electric energy to each spark plug (where the energy has a high voltage component capable of ionizing the plug gap to create an electrical discharge--the spark). The kernel of fuel burn that results grows so that in about 2.0 milliseconds the fuel-air mixture within the combustion chamber completely combusts. This time value required for complete combustion is constant for each particular engine throughout the operating speed range. As the engine speed increases, additional ignition timing advance must be provided so that at the end of 2 milliseconds, preferably at: about 9 degrees ATDC (after top dead center), the burn is complete.
Prechambers provide a "torch" ignition which burns much faster (300 ft./sec. as opposed to a 50 ft./sec. flame front velocity for spark-ignited engines). Pre-chambers therefore require less advance timing than typically needed must be set to complete the burn at 9 degrees ATDC. However, the torch burn rate changes as the main combustion mixture is made leaner, and greater ignition timing advance should be added to achieve maximum efficiency from the engine with reduced emissions. Because of the small ignition timing advance angles (e.g., 5 to 7.5 degrees) that are involved, the accuracy of timing is generally much more critical than in engines without pre-chambers.
Even though the use of pre-chambers to achieve more reliable, rapid and efficient combusion is known, it appears that no one in the past has achieved successful variable, accurate control of pre-chamber mixture to optimize pre-chamber and main chamber combustion over most or all conditions of engine operation.
The combustion of the air-fuel ratio in the pre-chamber is limited in the same way the mixture in the main chamber is limited. The pre-chamber works best when the mixture within it has a highly combustible 14.7:1 ratio. 16:1 (lean) and 13:1 (rich) air-fuel mixtures in the pre-chamber are practical limits of the range of pre-chamber mixture ratios, since the slower combustion speeds at those limits affect the ability of the pre-chamber to cause burning of the combustion chamber mixture. Missed burns can occur above or below these values. It is therefore generally desirable to control the pre-chamber mixture to always reside within these limits at or near stoichiometry. Such accurate control of pre-chamber mixture ratio typically requires an independent fueling capability for the pre-chamber. Techniques for separately fueling pre-chambers are generally known. Some exemplary prior art attempts to provide this function are discussed briefly below.
One exemplary prior art pre-chamber system has been disclosed in U.S. Pat. No. 3,919,985 to Yagi et al, which shows an internal combustion engine having a pre-chamber built into the engine head. The pre-chamber has a separate fueling capability that inserts a separate fuel-air mixture into the pre-chamber with a separate cam-operated valve. The pre-chamber mixture is ignited with a spark plug in the usual way.
U.S. Pat. Nos. 3,908,625 to Romy, 4,006,725, to Baczek et al, 4,218,993 to Blackburn and 4,248,189 to Barber et al, also all describe means to fuel a pre-chamber separately from the main combustion chamber.
U.S. Pat. No. 4,239,023 to Simko describes a means to use high pressure direct injection into both a pre-chamber and the main combustion chamber. This injection takes place during the compression stroke of the engine and, therefore, must utilize a high pressure injector.
U.S. Pat. No. 4,014,301 to Happel and assigned to Daimler-Benz Aktiengesellschaft, describes a dual fueling engine control whereby the pre-chamber is fueled separately from the main combustion chamber of the engine. A fuel injector is used to add fuel directly to the pre-chamber (which is built into the engine). The quantity of fuel so delivered is made to increase at idle speeds and to decrease at higher loads on the engine. The means shown to vary the pre-chamber fuel supply uses a mechanical arrangement on the engine distributor that varies the injector on-time as a function of RPM and engine load (vacuum). The fuel injector in the engine must be able to withstand the intense heat generated in the cylinder head and is costly to manufacture.
Other devices (shown, for example, in U.S. Pat. Nos. 4,071,001 to Goto and 4,085,713 to Noguchi et al) have made use of pre-chambers that catch a rich part of the intake air-fuel mixture in a trap-type of chamber.
Unfortunately, some significant problems arise in so controlling the prechamber mixture air-to-fuel ratio. Even the prior art techniques providing separate fueling of the pre-chamber fail to overcome these problems.
A pre-chamber is typically in direct fluid communication with the main chamber via at least one port for direct fluid transfer. Thus, as the piston moves upward during the compression stroke and compresses the gases (main chamber air-fuel mixture) in the cylinder, a portion of the compressed gases typically flows into the pre-chamber. This effect causes more than a slight dilution of the pre-chamber mixture, however. Most present-day automotive engines have compression ratios ranging from 8.5:1 to 12:1. Thus, an engine with a 10:1 compression ratio will fill the pre-chamber with approximately 90%, by volume, of the mixture in the engine cylinder, and only 10% by volume of the pre-chamber will contain the original mixture which was introduced into the pre-chamber before compression started!
None of the prior art teachings mentioned above teach or suggest a way to change or compensate the quantity of fuel in the pre-chamber mixture as the main chamber mixture is varied. As a result, the engine fails to respond adequately under some operating conditions even though the pre-chamber is fueled separately--and even though such separate fueling technique may be adjusted (as in the Daimler-Benz system) in response to change in engine operating parameters. The prior art pre-chamber devices all fail to keep the mixture in the pre-chamber at or near the ideal stoichiometric value as the main chamber mixture varies. This is a serious drawback since, as discussed above, the main chamber air-fuel mixture makes up a very large percentage of the pre-chamber volume during compression.
Of course, the main combustion chamber mixture must be variable to allow for normal acceleration during vehicle operations--even for lean burning engines. A practical automotive vehicle which is designed to have its engine run lean during steady state operation must satisfactorily cope with changes in engine speed, such as in acceleration during passing and in other everyday driving maneuvers. To accomplish this control, the main combustion chamber mixture must be made richer to prevent engine stumbling or partial stall. The pre-chamber mixture should therefore also be made leaner to prevent the compressed combined mixture of the main chamber mixture and the pre-chamber mixture from being too rich which, as discussed above, can hinder combustion.
The inability to accurately control the mixture in the pre-chamber as the mixture in the main chamber varies has prevented pre-chambers in the prior art from providing ignition enhancement over the entire engine operating range.
The present invention overcomes these problems by providing a lean-burn engine management system using pre-chamber technology that facilitates optimum combustion (and, if desired, lean-burn engine operation) over the entire operating range of the engine.
One feature of this invention is to provide a method and apparatus for promoting efficient lean-burning of an internal combustion engine utilizing electronic fuel injection and a combustion pre-chamber communicating with a main combustion chamber.
In a preferred exemplary embodiment of an internal combustion engine system in accordance with this invention, an ignition combustion pre-chamber is provided for igniting an air-fuel mixture in an internal combustion engine. The pre-chamber operates in conjunction with a microprocessor-based engine management system to maintain an optimum stoichiometric air-fuel ratio in the pre-chamber throughout the complete operating range of the engine.
In one embodiment, a one-way check valve admits a separate air-fuel mixture into the pre-chamber. After mixing with the portion of the air-fuel mixture of the main combustion chamber which necessarily enters the pre-chamber during the compression stroke of the engine, this pre-chamber mixture results in an ideal stoichiometric mixture receptive to quick ignition and faster flame front propagation throughout the main combustion chamber. If the main chamber mixture is a very lean (24:1) air-fuel ratio, by weight, the mixture introduced into the pre-chamber at the start of the compression stroke is made to be very rich, e.g., 4.5:1 air-fuel by weight, in order to provide a resulting pre-chamber mix of 14.7:1 after compression. If the mixture in the main chamber is richer than 14.7:1, a mixture leaner than 14.7:1 is introduced into the pre-chamber to provide a resulting pre-chamber mix near stoichiometry after the main chamber compression stroke.
In accordance with a further feature of the invention, the amount by which the pre-chamber mixture is enriched or leaned out (as appropriate) is calculated based on the actual (or estimated) main chamber mixture so as to provide a pre-chamber mixture at the instant of ignition (e.g., substantially after the main chamber combustion stroke has occurred) that is optimum for combusting.
In a further exemplary embodiment of the present invention, the pre-chamber is separated from the main combustion chamber by a further one-way check valve which prevents mixing of the separate air-fuel mixtures during the compression stroke. This allows a constant stoichiometric air-fuel ratio to be introduced into the pre-chamber and ignited while the main combustion chamber compression stroke is occurring. Fluid communication between the pre-chamber and the main chamber is then established to permit the pre-chamber flame front to enter the main chamber and ignite the mixture therein. Such in U.S. Pat. No. 3,710,764 to Jozlin, which fuels the pre-chamber with the main chamber mixture and then provides isolation of the pre-chamber from the main chamber during pre-chamber ignition so as to force the torch to entire the main chamber through an auxilliary port. This pre-chamber/main chamber isolation feature provided by the present is highly useful is permitting the pre-chamber to be fueled using a relatively unsophisticated air/fuel delivery system (e.g., a carburetor type system) having little or no capability to instantaneously adjust the pre-chamber mixture for different engine operating conditions and main chamber air-to-fuel ratio mixtures--while nevertheless providing nearly optimum burning of pre-chamber mixture (thus significantly enhancing main chamber combustion).
In accordance with yet another feature of the present invention, alternate gaseous fuels can be used with the pre-chamber and main combustion chamber where solenoid valves introduce a constant 100% concentration of gaseous fuel into the pre-chamber to provide the desired rich mixture for initial ignition.
Thus, one important feature of the present invention is to provide a pre-chamber fueling system which responds to changes in the main chamber fuel mixture to produce, over a range of practically all useful engine operating conditions, a combined mixture of compressed gases that is very combustible.
The present invention also provides a simple spark plug adaptor unit containing the pre-chamber that can screw into the normal spark plug hole--thus permitting the various other advantageous pre-chamber technology features of the invention to be provided in a retro-fitted fashion on existing automotive engines, thereby eliminating the requirement for extensive engine redesign and engine manufacturer retooling.
Another object of the present invention is to provide a one-way check valve in the fuel inlet of the pre-chamber which allows a one-way flow of an air-fuel mixture into the pre-chamber. A spark plug hole is provided wherein a spark plug can screw into the pre-chamber for the purpose of igniting the compressed air-fuel mixture at the usual time.
Another object of this invention is to provide a pre-chamber that is disconnected (isolated) from the main chamber during compression, power and exhaust strokes by using a second one-way check valve. The fuel-mixture introduced into the pre-chamber may then be always at stoichiometry (a constant 14.7:1 air-fuel ratio) to ensure optimum burning in the pre-chamber under all engine operation conditions.
The invention also provides utilization of alternate gaseous fuels in a pre-chamber arrangement.